The human lung is not fully matured but continues to develop during the first 6 years or even 15 years of life. Thus it is probable that severe respiratory disease of early infancy and childhood may affect the normal development of the lung and contribute to the onset of chronic lung disease in adult life. Although information on the functional development of the lung is important, such information during the first 6 years of life is unavailable because of obvious difficulties to study young patients. We propose to study respiratory mechanics in healthy subjects, anesthetized for elective surgery, between early postnatal days to young adulthood to get a complete profile of its functional development. Studies include lung volume with helium dilution, pressure volume curves of the lung and thorax, closing capacity with helium, and maximum expiratory flow volume curves with oxygen and helium-oxygen mixtures. Flow-volume curve will be produced during forced deflation of the lung. Instrumentation and safety of this apparatus has already been worked out. Similar studies will be carried out on patients in respiratory failure who are heavily sedated, paralyzed and mechanically ventilated in the intensive care unit. These studies have been approved by the Human Investigation Committee.